Various techniques for implementing electrical filters with active circuits, which use, in particular, integrated circuits in bipolar or MOS technology, are known.
Such filters no longer require the use of expensive and cumbersome inductors, yet they require the use of external capacitors, since high-value capacitors and active elements are difficult to integrate on the same chip. This problem has been partly resolved by implementing switched-capacitor filters in MOS technology, so that capacitors can also be integrated therein. That is made possible thanks to the easy integration of capacitors and switches typical of MOS technology.
Yet serious compatibility problems arise when an MOS circuit is to be inserted into a bipolar circuit, both owing to different input impedance values and to different breakdown voltages.
Unluckily, switched-capacitor filters cannot be easily implemented in bipolar technology, since the specific capacity is equal to about half that obtainable with MOS technology, which entails large area occupancy and difficult implementation of switches.
Integrated circuits capable of implementing analog filters in bipolar technology have been proposed in an article entitled "Integration of analog filters in a bipolar process", by J. O. Voorman et alii, IEEE Journal of Solid-state Circuits, Vol. SC-17, No. 4, August 1982.
The authors start with a classical LC network which is by using integrated capacitor and inductances simulated by integrated active circuits, comprising at least one integrated capacitor.
The circuit which simulates the inductance, implemented according to classical solutions, is dependent only on the integrated capacitor, not on the integrated resistances, hence frequency stability and accuracy exclusively depend on said capacitor.
However, filters thus implemented demand in their applications at frequencies of the order of KHz the integration of capacitors with high capacity values and different from each other, hence they have limited versalitity. In addition, also the resistance values must be high, to have high values of RC constants, and hence considerable space is taken up by their integration.
Another problem to be solved while designing the output stages of integrators or compensated operational amplifiers is the instability due to the presence of a zero with positive real value in the transfer function. In particular the problem arises when the current in the integrating or compensating capacitor is controlled by the collector of the voltage-amplifier transistor, driving the output stage. One way of overcoming the disadvantage is that of connecting a resistance in series with the capacitor, yet zero compensation is not highly accurate, since generally an over-compensation is effected for security reasons. This results in a zeroing value with a negative real part and a null imaginary part. Another way of overcoming the disadvantage is that of charging the capacitor through an emitter follower, which does not exclude the rise of oscillations due to the introduction of a further active element in the feedback loop.
Still another disadvantage presented by said circuits is due to limited voltage range of the signals which can be handled. In fact currents in transistors are to be kept low in order to have high collector impedance and hence reduce as much as possible the influence of the low-frequency parasitic pole.
Since resistances higher than a certain value are not feasible, voltages have to be kept low, and hence the signals which can be handled must present limited maximum voltages.